Soft-finish capacitor with TOP247Y

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I have a design with TOP247Y (already a bit older) where a small percentage of power supplies shows start up problems. In several documents a soft finish capacitor has been discussed. There are basically two variants. The basic one consists of just a capacitor which can range from 10 uF to 47 uF across TL431 (pins 3 and 2), see for example DI-41 (Design Idea 41). A more extended version adds a diode in series with the capacitor, and a resistor to created a discharge path. Once the capacitor is charged, the diode kind of isolates the capacitor from the circuit. See for example DI-43.

Is it possible that the basic circuit with just one capacitor can cause stability issues? What are the considerations to choose either the basic version or the more extended version?

Thanks and regards,
Henk

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Using the single capacitor option for "soft finish" requires use of a general purpose capacitor of value 1-10 uF (generally), depending on the particular application. A general purpose capacitor (examples are UCC KME series, Nichicon VZ series, or Panasonic NHG series) in that capacitance range will have an ESR of several tens of ohms that is swamped out by the cathode-anode impedance of the TL431 (~1 ohm). Use of a high quality low ESR capacitor or a ceramic capacitor for soft finish might cause stability issues unless a series resistor is introduced.

The more complicated diode-resistor-capacitor solution does not have this sensitivity to capacitor ESR, as the diode isolates the capacitor from the TL431 cathode during normal operation. The resistor will pull the capacitor to sufficiently high voltage such that the diode is reverse biased, isolating the capacitor from the TL431 cathode.

Both of these solutions actually regulate dV/dt during startup while the TL431 is still inactive. A higher startup dV/dt results in more current through the soft finish network, and hence the optocoupler LED, regulating the output voltage rate of rise. This allows a smooth takeover by the TL431 when the output voltage actually reaches the regulation set point.

Thanks for the quick answer. This is very helpful. With the simple solution, using a 22 uF Panasonic NHG capacitor, the supply seemed instable sometimes, but not always. Calculating ESR from tan(delta) I arrive at 7.25 ohm. Will try the more complicated solution.

You can either try the more complex soft finish option, or add some additional resistance in series with the soft finish capacitor. BTW, 22 uF seems rather large for a soft finish capacitor. what does your output voltage startup waveform look like with that value of soft finish cap?

The choice of 22 uF is based on what I often saw being used in PI documents. The values I have seen range from 10 uF to 47 uF, but mostly 22 uF or larger. I will use the more complex circuit which isolates the capacitor from the circuit after it has done its job.

It would be a better idea to start with about 1uF and work up. from there. Quite frankly, I've never used anything as large as 22uF in any of my designs. The more complex circuit will certainly work for you, but if you can get away with fewer parts..

In principle I agree with you that fewer parts is more attractive. However, I don't want to make it a trial and error, the fix should work guaranteed. I have gone through a number of documents on the Power website and never seen something as small as 1 uF. See overview below. With extended I mean the three component solution. Basic is just the capacitor.

AN-57. TOP247Y extended soft finish 10 μF
der34 TOP242P extended soft finish 10 μF
der 218 TOP260EN extended soft finish 10 μF
di43 TOP244Y extended soft finish 10 μF

TOP242.250 TOPSwitch GX family datasheet. TOP249Y Extended soft finish 22 μF

der95 TOP245Y basic soft finish 47 μF
di16 TOP246Y basic soft finish 22 μF
di41 TOP247Y basic soft finish 22 μF
di66 TOP246Y basic soft finish 10 μF
EPR13 TOP246 basic soft finish 22 μF

So it seems that indeed 22 μF is not necessary. I used the circuit with capacitor, 10 μF, 1N4148 and 15k resistor. The power supply is a 12V supply.